The present invention relates to a hydraulic control device suitable for a motor vehicle hydraulic braking circuit; more particularly the invention relates to a hydraulic control device comprising a master cylinder, at least one chamber of which is connected to the said braking circuit and which has its pressure controlled by the depression of a master piston under the action of a pedal, an auxiliary hydraulic circuit having a source of pressure, a jack, a piston of which cooperates with the said master piston to cause the depression of the latter, a distributor suitable for feeding the jack with auxiliary hydraulic pressure from the said source, the said distributor having a slide valve which is subjected to two opposing forces, one of which is developed by the auxiliary hydraulic pressure and the other of which is developed by the action of the pedal via means for transmitting force between this pedal and this slide valve, with the intervention of elastic means for simulating the pedal movement, and manual back-up means acting on the master piston, from the pedal, in the event of failure of the auxiliary circuit.
The master cylinder of the present invention is particularly suitable for an arrangement in which two independent braking circuits are provided, the master cylinder having two chambers which are isolated from one another and are respectively connected to the said braking circuits, and the pressure in each of these chambers being controlled by the depression of the master piston.
In hydraulic controls of this kind, the displacement stroke of the slide valve of the distributor, for increasing or reducing the intervention capacity of the auxiliary braking circuit in accordance with the pressure applied to the brake pedal by the driver is very small, for example of the order of a millimetre. It is for this reason that the elastic means for simulating the pedal movement are provided. In this manner, the driver has available a foot movement which is not reduced to such a small value, but which has an adequate value for driving comfort and safety, that is to say of the order of about ten centimetres, and gives the driver a feeling of total control over braking.
The hydraulic controls of this kind thus possess a sensitivity of response of the auxiliary braking circuit to the desire shown by the driver by means of the pedal, which results from the instantaneous value of the force exerted on the pedal rather than from the amplitude of depression of the pedal. The effect thus obtained is generally termed "force feed-back" and exhibits, inter alia, the advantage that it avoids a worrying blank movement of the pedal in the event of a break in one of the hydraulic braking circuits. This results in considerably enhanced driving safety.
However, the hydraulic controls of the above-mentioned type exhibit the disadvantage that, in the event of failure of the auxiliary hydraulic circuit, the driver must physically overcome not only the resistance offered by the braking circuits but also the resistance offered by the deformation of the elastic means for simulating the pedal movement. The effectiveness of the manual back-up in the event of failure of the auxiliary braking circuit is then reduced and even runs the risk of being jeopardised.
The present invention relates to a hydraulic control device in particular for a motor vehicle braking circuit, which control device is of the type described above but which does not exhibit this disadvantage, which is of simple and convenient construction and which makes it possible, if necessary, to use components of a common type, in its construction, such as conventional mastercylinder elements, a customary type of distributor, and the like.